This invention relates to a heavy load pneumatic radial tire used for vehicles such as trucks and buses.
Radial tires have recently been used for heavy load vehicles such as trucks and buses and have exhibited their excellent characteristics such as high speed performance, wear resistance and low fuel cost. To withstand a heavy load, the heavy load pneumatic radial tire includes a reinforcing belt of metal cords disposed at its tread in order to provide extremely high rigidity to the tread, and is used while air of a high internal pressure is charged into it.
In the heavy load pneumatic radial tire which has high rigidity at the tread and into which air of a high internal pressure is charged, however, there occurs the problem that shoulder off wear develops because a slip at both shoulders with respect to a road surface is greater than at the tread center. When the heavy load pneumatic radial tire runs on a road surface having wheel tracks, the tire receives an excessive external force from the road surface when it escapes from the wheel tracks. Accordingly, there occurs another problem that a steering wheel is steered in the wrong direction or in other words, the problem of so-called "wandering" occurs.
The problems described above which are inherent in the conventional heavy load pneumatic radial tires have been solved to some extents by forming thin grooves or sipes having a groove width of 1.5 to 5 mm and a groove depth 60 to 120% of the depth of main grooves at both shoulders in a tire circumferential direction. However, this counter-measure has not necessarily been sufficient and has not entirely been sufficient for the former problem, that is, for the shoulder-off wear.